USGI is correct that approximately 14.7 psig (standard atmospheric pressure) is the maximum differential pressure that will be applied to the stock if a perfect vacuum is achieved within the bag. But, you will not get close to a perfect vauum in the bag with a floor sweeper. That being said, phillydude is not entirely incorrect (except for the 1500 psi) when he notes that damage can be done.

Pressure (force applied per unit of area) can be a very counterintuitive condition if you are not accustomed to working with it. 1000 pounds of force sounds pretty nasty. Only 1 pound per square inch of pressure on a 33-inch by 33-inch surface sounds pretty benign - but either way the actal resultant force to be resisted is 1000 pounds. In addition, there are structural considerations to be considered that are even more critical than the pressure (vacuum) force considerations (i.e you couldn't damage an iron bar in a vacuum bag but you might damage a balsa wood model). Bottom line - applying 1 psig of pressure to a surface that can support only 1/2 psig of pressure will result in the same failure as applying 1500 psig.

I doubt that a vaccum bag would damage most assembled firearms but I do agree with phillydude to the extent you should look at what you put in the bag and if there are any thin and/or projecting wood sections that will be 'squeezed' by the bag you should take steps to support them before you evacuate the bag. Telescopic sights and batteries in scopes and lasers might be other issues to be considered. Maybe 99% of the time it's no problem but after you break it or permanently warp it it's too late to figure it out. JMHO ChipS